1. Field of the Invention
The invention relates to an apparatus and method for monitoring the mass flow rate of particulate solids, particularly, titanium dioxide.
2. Description of the Prior Art
In many chemical processes which include transporting particulate solids, it is important to continuously or intermittently monitor the solids' mass flow rate in order to properly maintain the process controls needed to obtain satisfactory product.
For example, during finish processing of titanium dioxide pigment, an accurate measurement of the mass flow rate of the titanium dioxide improves significantly the quality of the finished product.
Techniques presently used to determine mass flow include utilization of a rotary feeder system having a predetermined rate of rotation and inferring the mass flow rate from the rate of rotation. There are several problems with this technique, all of which result in inaccurate mass flow rate determinations. Although the volume of the feeder pockets is known, the pockets are not uniformly filled. Also, particle size, moisture content and other variables affect the mass transfer rate for a given revolution rate. Additionally, fine materials tend to be retained for a period of time within the feeder pockets and the surrounding casing as a result of pressure differentials.
U.S. Pat. No. 4,520,677 discloses a method and apparatus for indicating mass flow of a solid particulate material through a star wheel rotary feeder. Such feeders include a plurality of vanes extending radially from a rotating shaft, mounted within a housing, to form a plurality of rotating pockets which receive, transport through the housing, and subsequently discharge solid particulate material. The apparatus utilizes a source for discharging a radiation flux positioned within the rotating shaft. The radiation flux is discharged toward a detector, through the shaft and pockets containing the solid particulate material. By measuring the attenuation of the radiation flux at various points, e.g., when directed through a parallel vane, a filled pocket or a discharged pocket, a measurement of the mass can be calculated. However, this method loses accuracy because an estimated adjustment is needed to accommodate peaks in the radiation flux attenuation caused by the vanes being parallel in a star wheel rotary feeder.
It is therefore desirable to provide an apparatus and method for determining particulate solid mass flow rate which alleviate the discussed limitations. It is further desirable to provide an apparatus and system which is simple, reliable and which takes advantage of existing technology and components.